It is known in the prior art that ultrasonic energy may be coupled into an object using an ultrasonic transducer applied to the surface of the object. It is also known that a coupling material must be located between the transducer and the surface of the object if the soundwave is to be coupled into the object efficiently.
The conventional type coupling methods are not practicable under all circumstances. Thus, conventional coupling methods are considered not to be practical or capable of use under circumstances that the object, such as a bloom or slab of metal undergoing test is moving in an on-line plant application at a high rate of speed. Under these particular conditions, among other possible conditions, non-contact coupling techniques and methods generally are required to transmit and receive the ultrasonic signal.
Two well known techniques and methods of non-contact generation and reception of ultrasound utilize an electromagnetic acoustic transducer (EMAT) and a laser. Both of these non-contact techniques, however, are considered to present certain problems and disadvantages. For example, the EMAT requires a close proximity, possibly in the range of less than about 0.1 inch, between the material and transducer. In many industrial applications, a sensor will be subject to damage if positioned at such a close range of proximity to material undergoing test. Further, an EMAT can only be used on metal base materials, and the EMAT is a poor receiver of ultrasound. On the other hand, the laser which may be more distantly spaced from the material, for example, spaced at a distance possibly of several feet, requires for use product surface preparation and a high power confined beam to generate ultrasound efficiently. The use of high power lasers in a plant environment may create potential safety problems.
The prior art includes further disclosures of an air coupled ultrasonic transducer wherein the acoustic impedance of the transducing element is matched to the acoustic impedance of air. Thus, the prior art includes a paper, entitled "Ultrasonic Impedance Matching From Solids to Gases", by Lawrence C. Lynnworth, which describes that one quarter wavelength thickness matching layers may increase intensity of ultrasound transmitted from solids to gases. Lynnworth discloses the use of combined resonant- and nonresonant matching layers. The resonant layers are one quarter wavelength, and the nonresonant layers are some wavelength other than one quarter wavelength. In order to approach maximum gain, possible by impedance matching, Lynnworth describes new acoustic materials having impedances between the impedances of gases and solids. To this end, Lynnworth discusses matching layers of compressed gas and water. These matching layers are not considered suitable for durable industrial apparatus. It is also considered that the use of diaphragms to contain the fluids undoubtedly will reduce the efficiency that otherwise may have been realized by the Lynnworth apparatus.
Another paper of the prior art, entitled "Use of Piezoelectric Transducers for Contactless Ultrasonic Product Inspection", by V. I. Zaklynkovskii and G. T. Kartsev, describes an air coupled transducer for non-contact inspection of materials. According to the authors, inspection is carried out by coupling ultrasound using a single one quarter wavelength matching layer in front of the piezoelectric layer and a semi-infinite damping layer in back of the piezoelectric layer. The technique and apparatus is considered to suffer from low efficiency, and the low operating frequency of the apparatus, up to about 50 KHz, is also considered to be a drawback.
U.S. Pat. No. 3,928,777 to Frank Massa describes a transducer somewhat similar to the transducer described by Zaklynkovskii et al although having an operating frequency of up to about 280 KHz. According to Massa, a one quarter wavelength layer of potting compound is used as the matching layer. It is considered, however, that potting compounds, usually resilient in nature, are not efficient conductors of ultrasound. Further, the impedance of resilient potting compounds is significantly different from the impedance required for good matching.
A further disclosure of the prior art is U.S. Pat. No. 4,594,897 to Walter J. Bantz. In the patent, Bantz describes a transducer consisting of a layer of piezoelectric material and two matching layers. The piezoelectric layer operates at other than one half wavelength resonance. The first matching layer of the coupling medium also operates at other than one quarter wavelength while the second matching layer of the coupling medium operates at one quarter wavelength resonance at the operating frequency determined by the composite of the piezoelectric layer and the first layer. According to Bantz, the first matching layer consists of two layers of different materials. In order to operate at frequencies on the order of 500 KHz, the individual layers are required to be thin and of precise thickness throughout. While the Bantz apparatus operates substantially satisfactorily, the specific criteria of thickness and non-uniformity of the adhesive materials required to bond the four layers together may introduce a considerable measure of impedance mismatch and loss of efficiency.
A further patent of the prior art is U.S. Pat. No. 4,523,122 to Masayuki Tone, Tsutomu Yano and Koetsu Saito (hereafter "Tone et al.") which describes the matching of acoustic impedance of the piezoelectric source to air by applying either one or two matching layers of special materials, such as a porous polymer film or a composite material comprising thermally expanded resin microspheres to the transducer face. Tone et al. accomplish this by providing materials containing microspheres of specific size and distribution dispersed in the cured product. It is considered that the development of these special materials with specific impedance characteristics is time consuming and difficult, and it is known that porous materials exhibit high absorption of ultrasound. Thus, in all likelihood the impedance matching scheme of Tone et al. will sacrifice a substantial measure of transducer efficiency.
The prior art also provides a recognition that the velocity of ultrasonic waves is related to Young's modulus, and that there is a relationship between Young's modulus and the tensile strength of paper. This recognition may be drawn from several articles including an article "Plate Wave Resonance--A Contactless Test Method", by M. Luukkala, P. Heikkila and J. Surakka; an article "Ultrasonic Methods For Modulus Measurement In Paper", by Emmanuel P. Papadakis; an article "On-line Measurement Of Strength Characteristics Of A Moving Sheet", by Ming T. Lu; an article "Ultrasonic Plate Waves In Paper", by C. C. Habeger, R. W. Mann and G. A. Baum; and U.S. Pat. No. 4,291,557 to G. A. Baum and C. C. Habeger. The Baum et al. patent also describes a method of monitoring paper strength in real-time during the papermaking process. The method is carried out by mounting a transducer assembly to a wheel/roll. The transducer assembly is mounted on the papermaking machine so that it contacts the paper web to excite and detect plate waves of a specific frequency. While the method demonstrates that an on-line measurement of paper strength is possible the method is not without its disadvantages. To this end, the system consists of a large mechanical assembly which contacts the paper web. Therefore, it is considered necessary to limit use of the method in testing high strength papers, such as Kraft paper. In addition it is considered difficult to maintain synchronization between the transmit and receive transducer assemblies to obtain reliable long term mechanical/electrical operation, particularly when the wheel/roll must rotate at line speeds up to about 6000 feet per minute.
The present invention distinguishes from the disclosures of the patents and articles discussed above in that the ultrasonic transducer configuration of the invention provides greater efficiency of operation using impedance matching materials that are readily available. To this end, the invention uses two matching layers, each consisting of a single layer capable of vibrating in a thickness mode of vibration or in multiple modes of vibration. As the invention will be discussed, the first matching layer is intended to match the impedance of the piezoelectric layer to that of the second layer, providing an ultrasonic transducer having a higher efficiency at an operating frequency up to about 500 KHz.
The impedance matching techniques addressed in the disclosures of the patents and articles couple energy to air from a piezoelectric layer vibrating primarily in the thickness mode. The invention, on the other hand relates to a piezoelectric layer that vibrates at multiple modes at a single frequency. It has been found that significantly higher energy levels may be coupled to air when a piezoelectric layer vibrating in multiple modes is bonded to the impedance matching layer. A transducer of almost twice the efficiency of transducers operating only in the thickness mode has been demonstrated in effective operating ranges up to about 170 KHz.